The present inventions relates to an optical element forming metal mold to manufacture an optical element requires a sub-micron accuracy by injection mold of resin and a manufacturing method of the optical element forming metal mold.
For injection molding of resin A, a metal mold using metallic material such as a steel has been used. In rent years, a high degree of sub-micron accuracy has been required for the optical elements due to miniaturization and precision optical element.
To form a precision optical element, there is suggested an optical element forming metal mold (for example, Unexamined Japanese Patent Application Publication No. 2002-96335) having a heat insulation layer and a surface forming layer laminated on a surface of a core stainless steel mold, where on a surface of a core mold, representing a metal mold to form one surface of the optical element, a ceramic series material is sprayed by thermal spraying to laminate a heat insulation layer on the metal mold base member, then a surface forming layer is formed on the heat insulation layer thereof by electroless deposition of a nonferrous material. Here, thermal spraying is a laminating method where a material in a shape of powder is melt in a high temperature, a melt material is sprayed onto a subject to be laminated, and the material is cooled and set so as to laminate layers on the subject.
However, in such an optical element forming metal mold, by repeating temperature rise and fall for forming the optical elements, there is a possibility of layer amotion. In particular, between the heat insulation layer of a ceramic series material and the surface forming layer of a nonferrous metal material amotion tends to occur due to a difference of thermal expansion coefficients. There is a possibility that local amotion causes an imperceptible distortion and a miss alignment of the surface forming layer, and deteriorates an accuracy of shape of the product.
Therefore, there has been suggested an optical element forming metal mold where an intermediated layer made of a material having an affinity for both the surface forming layer and the heat insulation layer is formed on the whole area of the heat insulation layer by thermal spraying.
However, since the material of the intermediate layer is selected by taking account of the affinity of both the heat insulation layer and the surface forming layer, it cannot be the best in the affinity of both the heat insulation layer and the intermediate layer. Therefore, as a conventional optical element forming metal mold, in case the intermediated layer is laminated on the whole area of the heat insulation layer by thermal spraying, when the material of the intermediate layer is cooled down and sets, the stress amotion occurs between the heat insulation layer and the intermediated layer thus a desired adhesiveness may not be obtained.
If the amotion occurs as above, optical performance of the optical element is deteriorated by distortion of the optical element which is formed by the deformed surface forming layer where a gap created by amotion is collapsed by a large pressure applied to form the optical element. Further, since the deformation of the amotion portion caused by the pressure often returns to an original shape while the pressure is not applied, a method to carry out precision machining after the deformation is actively created by design, cannot be utilized.